It might seem like there are perks to exercising on an empty stomach, like burning more fat per workout, but the downsides greatly outweigh the benefits. Here’s why you’re probably better off fueling up before you go for a run or crank out some super sets at the gym.

Why the extra fat burn? As sports dietetics specialist Kelly Pritchett, Ph.D., R.D., puts it, your body is switching fuel sources. To perform intense physical actions like long-distance running or lifting weights, your body has to burn glycogen, or your stored up carbohydrates. If your body runs of out glycogen reserves—like it would in a hungry, fasted state—it has to use something else to keep you going. In this case, all that excess fat you’re hoping to get rid of.

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But there’s a catch—well, a few of them. For starters, your body isn’t a fan of being starved, and it likes having fat stores. When you burn fat rapidly, your body begins to adjust your metabolism to compensate for that loss. Basically, it goes into a kind of survival mode and starts to burn fewer calories, says Pritchett. By burning so much fat, your body thinks it needs to store more of it when eat your next meal, completely counteracting those fat burning benefits. And as another study in the American Journal of Clinical Nutrition found, extended periods of fasting can lead to a drop in resting metabolic rates. So there’s no real upsides to foregoing food in this type of scenario.

Furthermore, fasted exercise can cause you to shed some muscle in addition to fat under some circumstances. If your body has burned through its glycogen fuel stores, it may also obtain energy by breaking down muscle proteins in addition to those fats. Intense training always breaks down muscle so it can get stronger through protein synthesis, but doing so while fasted harvests more muscle sooner, making it more difficult to recuperate the lost mass.

That said, this only occurs when you increase the intensity of your workout beyond your normal routine. If, say, you were to do a normal workout first thing in the morning, your muscles would probably still have sufficient glycogen stores leftover from your previous meal. And according to one study from the Journal of Applied Physiology, being in a fasted state doesn’t increase or decrease your physical output or exertion during typical, submaximal exercise. Essentially, whether you workout hungry or not, you can do your usual thing at the usual intensity without worrying about muscle loss.

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That can lead to another problem, though. While you could get through a workout fasted, you won’t have the energy to push yourself harder, and thus, it’s unlikely you’ll be able to improve at the same rate were you to eat first. A separate study, published in the Journal of Science and Medicine In Sport, suggests that fasted training significantly underperforms fed training when it comes to maximal exercise, or reaching for new personal bests. Pushing yourself is what leads to increased muscle mass and faster run times, so you want to do it as much as you can. When it’s all said and done, eating before a workout can actually increase your metabolism in the long run.